The manufacture of certain resonant devices such as waveguides for particle accelerators and other high power microwave devices is a cumbersome and expensive process, because these devices require that a plurality of modules be interconnected by joints that simultaneously provide precise internal geometries, unbroken RF shielding, and a high vacuum seal. A typical device of this type might include a set of cylindrical resonator cavities connected together with coupling holes that are machined out of copper and then brazed together to achieve both a good vacuum joint as well as good RF contact. This brazed joint process is expensive, and is prone to manufacturing errors, whereby the desired resonant internal geometry is not obtained with sufficient precision or a sufficient vacuum seal is not achieved. Furthermore, if there is a mistake in the brazing process, it is virtually impossible to unbraze and rebraze the assembly. Moreover, thermal stress experienced by copper during brazing increases the probability of electrical breakdown.
Some attempts have been made to construct such resonant devices without brazing. In one case, resonant copper cells were clamped together, whereby an externally applied structure was used to apply and maintain the clamping pressure required to provide good RF contact. The vacuum seal in this case was provided by a separate, external stainless steel container. Accordingly, in this example the RF and vacuum joints were separated, and the resulting assembly was bulky and costly.
Another approach that has been tried is to bolt copper cells together with a softer, annealed copper gasket sandwiched in between the cells. Under ideal conditions, such an assembly can provides a simultaneous RF and vacuum joint that performs well. However, crushing a copper gasket between copper flanges is a delicate procedure, the reliability of which is uncertain. In particular, the relative softness of copper flanges poses a high risk of distorting the internal dimensions of the cells when the flanges are pressed against the gaskets. Also, disassembly and reassembly of such an assembly runs the risk of vacuum and/or RF leaks being introduced.
So-called “conflat” vacuum seal technology is well known to provide high quality vacuum seals that can be safely disassembled and reassembled with almost no degradation. These vacuum seals rely on the joining together of opposing hard, stainless steel flanges, whereby the opposing faces of the flanges include knife edges configured to bite into a thin, soft copper gasket that is sandwiched in between the flanges. However, while the conflat approach provides highly reliable vacuum seals, it is not able to provide reproducible internal geometries that would be sufficiently precise to satisfy the requirements of resonators and waveguides for particle accelerators and other such high power microwave devices.
Similarly, the Stanford Linear Accelerator Center (SLAC) and Compact Linear Collider (CLIC) projects developed various types of crush-seal RF waveguide flanges that featured rectangular lips rather than a waveguide and were able to provide vacuum seals while simultaneously providing good RF contact. However, the rectangular lips used in these designs do not provide a precision crushing thickness. For simple, non-resonant waveguide applications this is not important, but utilizing the same technology for constructing resonators would be problematic, because there would be an unacceptable variability of the resonator length. Furthermore, the situation would be even worse for accelerators, which are essentially arrays of coupled resonators, because the length error would accumulate.
What is needed, therefore, is an unbrazed waveguide module joint structure that provides a low cost joint having precisely controlled internal geometry combined with a reliable vacuum seal and high quality RF shielding, whereby the joint structure can be readily disassembled and reassembled with substantially no degradation.